GaN semiconductor is used in order to manufacture a blue or white LED invented currently. A GaN single crystal wafer is basically needed as a substrate to grow a GaN semiconductor using a CVD method. However, the GaN single crystal is difficult to grow, a GaN single crystal growth method, which can be put into practical use, has not been developed yet.
Meanwhile, Japanese Nakamura has produced a blue LED by growing a GaN single crystal on a sapphire wafer, and has succeeded in putting the blue LED into practical use. Many crystallographers have made efforts to grow the GaN single crystal for past 20 years or so, but have failed in the development of a grow method having an economical efficiency. Thus, it is naturally recognized that a sapphire (Al2O3) single crystal wafer should be used to fabricate a blue or white LED, and the demand for the sapphire single crystal wafer is explosively increasing.
A sapphire single crystal can be grown by various growth methods such as a Berneuil process, a hydrothermal process, a Czochralski process, a heat exchange method, a Kyropoulos method, an EFG process, etc. But, among them, a method suitable for growth of a sapphire single crystal having the quality and size enough to be used as a substrate for LEDs can include the heat exchange method and the Kyropoulos method. In addition, a c-plane sapphire wafer is used to fabricate the LEDs. It is preferable to manufacture a sapphire having a long cylindrical shape along a c-axis in terms of a yield in order to manufacture the c-plane sapphire wafer.
The Czochralski process is desirable for growth of a cylindrical single crystal. However, since the sapphire single crystal is difficult to grow in a c-axial direction, an ingot grown by the Czochralski process is mainly grown in an a-axis and is core-drilled vertically to form a cylindrical shape in a c-axis as shown in FIG. 1a. Then, the cored cylindrical ingot is sliced to produce a wafer, thereby resulting in a considerable decrease in a yield (up to a maximum of 30%).
Therefore, the Kyropoulos method is applied which can obtain an ingot having a short thick cylindrical shape rather than an elongated cylindrical shape in order to improve a yield. Also, it is regarded that the quality of crystals grown by this method is superior to that of crystals grown by the Czochralski method. However, as the diameter of the sapphire wafer is currently increased, for example, from 2 inch to 4 inch, a yield of the single crystal grown by this Kyropoulos method is no more than 32% or so. Moreover, there occurs a problem in that if a larger wafer is used, the Kyropoulos method is difficult to apply.
As an original patent related with the growth of a sapphire single crystal using a heat exchanger method, U.S. Pat. No. 3,898,051 (issued on Aug. 5, 1975) discloses that a short cylindrical crystal grown as shown in FIG. 1b, it has a yield (32-34%) similar to that in the Kyropoulos method. But it is known that when a square crucible is used as shown in FIG. 1c, a yield (about 70%) of the crystal can be improved greatly. However, there may occur a problem in that if the shape of the crucible is changed into a long shape such as a rectangular shape, it is not easy to uniformly maintain the internal temperature of the crucible. The reason for this is because in the case where a heater is surroundingly installed around the outer wall of the crucible, the crucible is the lowest in temperature at the center thereof and is gradually increases in the temperature toward the edge of the crucible from the center of the crucible. In other words, a seed crystal has a higher temperature at both ends thereof than at the central portion thereof.
In order to address and solve the above problem, Korean Patent Registration No. 0428699 (Application No. 10-2001-0011553) proposes a method which can provide a desired vertical and horizontal gradient freezing (VHGF) by using a long crucible and varying the width and thickness of a heater. However, in case of employing a long rod-like seed crystal, there is caused a problem in that the temperature according to the length of the seed crystal cannot be uniformly maintained. In particular, if the length of the crucible increases, it will be further difficult to uniformly maintain the temperature in the horizontal direction of the crucible including the temperature of both ends and central portion of the crucible. In addition, there is caused a problem in that since the state of an insulator is changed depending on the use frequency and the use period at a temperature of more than 2000° C., although the internal temperature of the crucible is uniformly maintained by adjusting the width and thickness of the heater, it is changed along with an increase in the use period and use frequency. In this manner, if the temperature in the horizontal direction of the crucible is not uniformly maintained, particularly, in case of using a long rod-like seed crystal, the seed crystal placed at the central bottom of the crucible may melt depending on a position in a longitudinal direction of the crucible or a raw material charged into the crucible may not be melted. If such a phenomenon occurs, a single crystal cannot be grown. Also, the seed crystal is not completely melted and does not have a uniform shape, thereby deteriorating the quality of the grown crystal.
Thus, the prevent inventors have found that (1) in the case where a plurality of heaters dividedly disposed at the outer side of a crucible is independently operated, the horizontal temperature at the inside of the crucible can be uniformly maintained, that (2) in the case where a heater including a lateral heating element and a connecting heating element is used, a vertical temperature gradient can be obtained and the number of electrodes can be reduced, that (3) in the case where the bottom of the crucible is formed concavely inwardly or convexly outwardly, the non-melting of a sapphire scrap or the complete melting of the seed crystal can be prevented, and that (4) in the case where an annealing step is performed after the completion of the crystal growth, the quality of a single crystal can be improved, thereby completing the present invention.